Method of wafer patterning for reducing edge exclusion zone

ABSTRACT

A method includes steps of: (a) providing a wafer on which a film has been deposited; (b) exposing an annular area in an edge exclusion zone of the wafer to radiation having a wavelength suitable for patterning the film in the annular area; and (c) modulating the radiation while exposing the annular area to form a pattern in the film in the annular area.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is directed to methods of manufacturing integratedcircuits. More specifically, but without limitation thereto, the presentinvention is directed to a method of optimizing die yield in a siliconwafer.

2. Description of the Prior Art

In the manufacture of integrated circuit devices, a silicon wafer istypically partitioned into die or dice each having an identicalarrangement of semiconductor structures. The die are formed on thesilicon wafer by a photolithography tool, called a stepper. The stepperprints the die in groups, called shots, on the surface of the siliconwafer. Photo resist films are deposited and etched on the wafer toexpose specific areas of the die to various manufacturing processes. Theremoval of the buildup of the resist films at the edge of the waferresult in an unusable space on the edge of the wafer called the edgeexclusion zone. The number of die formed in the usable area of thesilicon wafer that perform satisfactorily to design specifications iscalled the wafer yield.

SUMMARY OF THE INVENTION

In one embodiment, an apparatus includes:

an edge expose unit for exposing an annular area in an edge exclusionzone of a wafer to radiation having a wavelength suitable for removing afilm from the wafer in the annular area; and

a radiation modulator coupled to the edge expose unit for modulating theradiation to pattern the film in the annular area.

In another embodiment, a method includes steps of:

(a) providing a wafer on which a film has been deposited;

(b) exposing an annular area in an edge exclusion zone of the wafer toradiation having a wavelength suitable for patterning the film in theannular area; and

(c) modulating the radiation while exposing the annular area to form apattern in the film in the annular area.

BRIEF DESCRIPTION OF THE DRAWINGS

The embodiments described herein are illustrated by way of example andnot limitation in the accompanying figures, in which like referencesindicate similar elements throughout the several views of the drawings,and in which:

FIG. 1 illustrates a wafer layout of the prior art;

FIG. 2 illustrates a typical wafer edge exposure plan for the waferlayout of FIG. 1;

FIG. 3 illustrates a top view of die clipping for a wafer edge settingof 3 millimeters;

FIG. 4 illustrates a top view of die clipping for a wafer edge settingof 2 millimeters;

FIG. 5 illustrates a plot of wafer planarization of the wafer layout ofFIG. 1 after a chemical mechanical process;

FIG. 6 illustrates a top view of a wafer edge exposure apparatus of theprior art;

FIG. 7 illustrates a side view of the wafer edge exposure apparatus ofFIG. 6;

FIG. 8 illustrates a top view of an improved wafer edge exposureapparatus;

FIG. 9 illustrates a side view of the wafer edge exposure apparatus ofFIG. 8 with temporal radiation modulation;

FIG. 10 illustrates a top view of a wafer layout generated by the edgeexpose unit of FIGS. 8 and 9;

FIG. 11 illustrates a magnified view of the edge exclusion zone in thewafer layout of FIG. 10;

FIG. 12 illustrates a side view of the wafer edge exposure apparatus ofFIG. 8 with spatial radiation modulation;

FIG. 13 illustrates a top view of a wafer layout generated by the waferedge exposure apparatus of FIG. 12;

FIG. 14 illustrates a magnified view of the edge zone in the waferlayout of FIG. 13;

FIG. 15 illustrates a magnified view of an edge exclusion zone using anedge expose unit with both temporal and spatial modulation;

FIG. 16 illustrates a top view of a wafer layout of the prior art withdummy shots; and

FIG. 17 illustrates a flow chart of a method of wafer patterning toreduce the edge exclusion zone.

Elements in the figures are illustrated for simplicity and clarity andhave not necessarily been drawn to scale. For example, the dimensions ofsome of the elements in the figures may be exaggerated relative to otherelements to help to improve understanding of the following descriptionof the illustrated embodiments.

DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

Achieving the maximum usable wafer area and the highest wafer yield iscritical to cost effective manufacturing of integrated circuits.Typically, a stack of etch resist films called an edge bead builds up onthe wafer edge. The edge bead causes defects in the die that are locatednear the wafer edge. Previous methods for removing the edge bead includea combination of solvent dispense edge bead remover (EBR) and a waferedge expose (WEE) process. Recently, additional techniques such as wetedge etching and edge scrub processes have been implemented as well.

The previous methods described above for removing or controlling filmsdeposited on wafers to reduce the number of defective die require somespace at the edge of the wafer. Initially, the industry standard for thenon-yielding zone at the edge of the wafer, called the edge exclusionzone, was four millimeters or more. As improved methods for controllingfilms became available, it has been possible to reduce the edgeexclusion zone to about three millimeters and even two millimeters. Aproposed industry standard has called for a further reduction of theedge exclusion zone to one millimeter by 2006.

FIG. 1 illustrates a wafer layout 100 of the prior art. Shown in FIG. 1are a silicon wafer 102, a wafer center 104, shots 108, completelypatterned die 110, a wafer notch 112, and an edge exclusion zone 114.

The wafer layout software generates a shot map of the die locations onthe wafer according to well known techniques. The shot map is thenoverlaid on the silicon wafer. In this example, each of the shots 108contains up to 20 die 110 arranged in a 4×5 rectangle.

Because die that extend into the edge exclusion zone 114 will be onlypartially patterned and will not yield, the shots 108 near the edge ofthe wafer 102 contain fewer completely patterned die 110 than the totalnumber of 20 in the available shot capacity. The edge exclusion radiusthat is used to determine the wafer layout 100 and the number of usabledie sites on the wafer 102, also known as gross die per wafer, is notactually the radius at which the films constituting the edge bead areremoved. This is because the physical edge of the films must be closerto the edge of the wafer 102 than the die 110 to avoid defects in thedie 110 that are closest to the radius of the edge exclusion zone. Inaddition, certain films may not be allowed to overlap an adjacent filmin the film stack to avoid particle defects, so that some films in thestack have a larger edge radius in the edge exclusion zone than thefilms above.

FIG. 2 illustrates a typical wafer edge exposure plan 200 for the waferlayout of FIG. 1. Shown in FIG. 2 are BARC layers 202, dark field waferedge expose layers 204, light field wafer edge expose layers 206, and acontact mask 208.

The Backside Anti-Reflective Coating (BARC) layers 202 are an organiccoating used to reduce interference from reflections during opticallithography.

The dark field wafer edge expose layers 204 are semiconductor layersthat have only a very little pattern. The chrome mask used has only asmall opening that allows light to pass through, giving the appearanceof dark layers.

The light field wafer edge expose layers 206 are semiconductor layersthat are mostly patterned lines. The chrome mask used has a large numberof openings that allow light to pass through, giving the appearance oflight layers.

The contact mask 208 is used to pattern the lower layers duringphotolithography.

Control of the edge removal settings with currently available equipmentis typically about +/−0.2 millimeters. To ensure that the BARC layers202, the dark field wafer edge expose layers 204, the bright field waferedge expose layers 206, and the contact mask 208 do not overlap, eachedge radius setting must be separated by at least 0.4 millimeters withan additional 0.4 millimeters to allow for the curvature of the waferedge. In this example, at least four non-overlapping settings areneeded, which adds up to a minimum of at least 1.6 millimeters requiredfor the physical edge settings.

The value of reducing the edge exclusion zone by one millimeter isconsiderable, as illustrated below.

FIG. 3 illustrates a top view 300 of die clipping for a wafer edgesetting of 3 millimeters. Shown in FIG. 3 are a 3 millimeter wafer edge302 and a die 304.

As shown in FIG. 3, the 3 millimeter wafer edge 302 results in clippingthe corner of the die 304, rendering it unusable.

FIG. 4 illustrates a top view 400 of die clipping for a wafer edgesetting of 2 millimeters. Shown in FIG. 4 are a 2 millimeter wafer edge402 and a die 304.

As shown in FIG. 4, the 2 millimeter wafer edge 402 avoids clipping thecorner of the die 304, rendering it usable. The difference between a 3millimeter edge setting and a 2 millimeter edge setting may be worth anadditional 10 to 30 die per wafer, depending on the die size and thewafer diameter. The increase in gross die per wafer represents asignificant financial value for a typical wafer fabrication facility.

Removing edge films by removing the etch resist and exposing the filmsto an etch process or by direct edge film etching improves wafer yieldsby avoiding stacking of films that have incompatible coefficients ofthermal expansion or poor adhesion with other films. In relatively largeareas of films such as in the edge exclusion zone, the films are moresusceptible to particle flaking during processes performed at hightemperatures and/or in a vacuum, resulting in particle defects.

A consequence of removing the edge films is the creation of a sharp stepat the edge exclusion zone. This step adversely affects the chemicalmechanical process (CMP) that forms a uniformly plane surface across thewafer. Each chemical mechanical process has a characteristicplanarization length, that is, the distance from the edge of a sharpstep before the desired thickness and planarity of the wafer isachieved. The sharp step at the edge exclusion zone creates a ring ofpoor planarization that extends about one millimeter from the step.

FIG. 5 illustrates a plot 500 of wafer planarization of the wafer layoutof FIG. 1 after a chemical mechanical process. In FIG. 5, the net effectof the edge steps interacting with the chemical mechanical process isthat the best possible edge setting of 1.6 millimeters in the example ofFIG. 2 plus one millimeter for the chemical mechanical processingplanarization length results in a minimum realizable edge exclusion zoneof 2.6 millimeters.

FIG. 6 illustrates a top view 600 of a wafer edge exposure apparatus ofthe prior art. Shown in FIG. 6 are a wafer 602 and an edge expose unit604.

In FIG. 6, the wafer 602 is rotated under the edge expose unit 604 toexpose the resist film around the edge of the wafer 602 to radiationhaving a suitable wavelength for removing the resist, for example,ultraviolet light. Typically, the wafer 602 is mounted on a rotatingtable. The exposed resist is then removed according to well knownprocesses for developing and rinsing away the resist film.

FIG. 7 illustrates a side view 700 of the wafer edge exposure apparatusof FIG. 6. Shown in FIG. 7 are a wafer 602, an edge expose unit 604, aresist film 702, an edge bead 704, ultraviolet radiation 706, and anedge exclusion zone 708.

In FIG. 7, a narrow line extending through the edge exclusion zone 708is exposed to the ultraviolet radiation 706 as the wafer 602 is rotatedunder the edge expose unit 604. The rotation of the wafer 602 results inthe exposure of the annular edge exclusion zone 708. The exposed resistis then removed from the edge exclusion zone 708 by, for example, asolvent rinse.

To achieve the goal of a one millimeter edge exclusion zone, control ofthe edge film removal processes would have to be improved from +/−0.2millimeters to +/−0.1 millimeters, and the chemical mechanicalprocessing planarization length would have to be reduced from onemillimeter to 0.2 millimeter. Another way to reduce the edge exclusionzone is to modify a standard wafer edge exposure apparatus to create adummy pattern in the edge exclusion zone so that the planarizationlength may be shortened to the stepped edges of the film layers.

In one embodiment, an apparatus for wafer patterning to reduce the sizeof the edge exclusion zone includes:

an edge expose unit for exposing an annular area in an edge zone of awafer to radiation having a wavelength suitable for removing a film fromthe wafer in the annular area; and

a radiation modulator coupled to the edge expose unit for modulating theradiation to pattern the film in the annular area.

To avoid the abruptness of the stepped layers following the bare edge ofthe edge exclusion zone 708 that adversely affects the chemicalmechanical processing, a dummy pattern may be created in the edgeexclusion zone as follows.

FIG. 8 illustrates a top view 800 of an improved wafer edge exposureapparatus. Shown in FIG. 800 are a wafer 802 and an edge expose unit804.

The wafer 802 may be rotated under the edge expose unit 804 according tothe same well known techniques used in the prior art for the wafer edgeexposure apparatus of FIG. 6. Alternatively, the edge expose unit 804may rotate around the wafer 802 according to well known mechanicaltechniques.

FIG. 9 illustrates a side view 900 of the wafer edge exposure apparatusof FIG. 8 with temporal radiation modulation. Shown in FIG. 9 are aresist film 702, an edge bead 704, ultraviolet radiation 706, a wafer802, an edge expose unit 804, an edge exclusion zone 902, a radiationmodulator 906, and temporally modulated ultraviolet radiation 908.

In FIG. 9, the edge bead 704 is exposed to the constant beam ofultraviolet radiation 706, for example, in the same manner as in FIG. 7.In the edge exclusion zone 902, however, the radiation modulator 906generates a pattern of lines, for example, by an electronic strobe or arotating shutter that is synchronized with the rotation of the wafer802.

FIG. 10 illustrates a top view 1000 of a wafer layout generated by theedge expose unit 804 of FIGS. 8 and 9. Shown in FIG. 10 are a wafer 802and an edge exclusion zone 902. The temporally modulated ultravioletradiation 908 in the edge exclusion zone 902 results in a pattern oflines that may be used for planarization by chemical mechanical process.

FIG. 11 illustrates a magnified top view of the edge exclusion zone 902in the wafer layout of FIG. 10. Shown in FIG. 11 are a wafer 802, anedge exclusion zone 902, and resist lines 1102.

In FIG. 11, the edge exclusion zone 902 of the wafer 802 has beenexposed to the temporally modulated ultraviolet radiation 908, and theexposed resist has been removed, leaving the pattern of radial resistlines 1102 between the edge of the wafer 802 where the edge bead 704 wasremoved and the usable portion of the wafer 802.

In another embodiment, the wafer edge exposure apparatus of FIG. 8 maybe used to generate a pattern of circular resist lines.

FIG. 12 illustrates a side view 1200 of the wafer edge exposureapparatus of FIG. 8 with spatial radiation modulation. Shown in FIG. 12are a resist film 702, an edge bead 704, ultraviolet radiation 706, awafer 802, an edge exclusion zone 1202, a radiation modulator 1206,spatially modulated ultraviolet radiation 1208, and an edge expose unit1210.

In FIG. 12, the edge bead 704 is exposed to the constant beam ofultraviolet radiation 706, for example, in the same manner as in FIG. 9.In the edge exclusion zone 1202, however, the radiation modulator 1206generates a pattern of resist circles as the wafer 802 is rotated. Inthis example, the radiation modulator 1206 includes a mask thatalternately passes and blocks the spatially modulated ultravioletradiation 1208 from the edge expose unit 1210.

FIG. 13 illustrates a top view 1300 of a wafer layout generated by theedge expose unit 1210 of FIG. 12. Shown in FIG. 13 are a wafer 802, andan edge exclusion zone 1202. The spatially modulated ultravioletradiation 1208 in the edge exclusion zone 1202 results in a pattern ofresist circles that may be used for planarization by a standard chemicalmechanical process.

FIG. 14 illustrates a magnified top view of the edge exclusion zone 1202in the wafer layout of FIG. 13. Shown in FIG. 14 are a wafer 802, anedge exclusion zone 1202, and resist circles 1402.

In FIG. 14, the edge exclusion zone 1202 of the wafer 802 has beenexposed to the spatially modulated ultraviolet radiation 1208, and theexposed resist has been removed, leaving the pattern of resist circles1402 between the edge of the wafer 802 where the edge bead 704 wasremoved and the usable portion of the wafer 802.

In another embodiment, both spatial and temporal radiation modulationmay be used to generate a checkerboard resist pattern in the edgeexclusion zone.

FIG. 15 illustrates a magnified top view 1500 of an edge exclusion zoneusing an edge expose unit with both temporal and spatial modulation.Shown in FIG. 15 are a wafer 802, an edge exclusion zone 1502, a resistpattern 1504, and temporally and spatially modulated ultravioletradiation 1506.

In FIG. 15, the edge exclusion zone 1202 of the wafer 802 has beenexposed to the temporally and spatially modulated ultraviolet radiation1506, and the exposed resist has been removed, leaving the checkerboardresist pattern 1504 between the edge of the wafer 802 where the edgebead 704 was removed and the usable portion of the wafer 802. Otherpatterns for populating the edge exclusion zone using temporal andspatial modulation of radiation from the edge expose unit and othertypes of radiation suitable for patterning wafers may be used topractice various embodiments within the scope of the appended claims.

An advantage of the apparatus for wafer patterning to reduce the size ofthe edge exclusion zone described above is that patterning the films inthe edge exclusion zone often relieves the stress between film layersthat may contribute to particle defects. Also, patterning the films inthe edge exclusion zone may enable other strategies for edge filmcontrol that require less space on the edge of the wafer.

Another advantage of the apparatus for wafer patterning to reduce thesize of the edge exclusion zone described above is that the practice ofplacing dummy exposures around the edge of a wafer to improve thechemical mechanical process may be avoided, thereby reducing the usagetime required from highly expensive photolithography equipment for dummyshots.

FIG. 16 illustrates a top view 1600 of a wafer layout of the prior artwith dummy shots. Shown in FIG. 16 are a silicon wafer 102, a wafercenter 104, shots 108, die 110, a wafer notch 112, an edge exclusionzone 114, and dummy shots 1602.

In FIG. 16, there are 16 dummy shots 1602 and 59 shots 108. The dummyshots 1602 are included around the edge of the wafer 102 to provide aresist pattern that extends to the edge exclusion zone 114, therebyensuring that yieldable planarization is achieved for the die 110 thatlie inside the usable wafer area. Although the dummy shots 1602constitute 21 percent of the total shots required from thephotolithography equipment, the dummy shots 1602 do not directly produceusable die, because a portion of the die inside the dummy shots 1602overlap the edge exclusion zone 114. By using the wafer patterningapparatus described above to achieve yieldable planarization for theusable die 110 that lie near the edge of the wafer 102 instead of thedummy shots 1602, a substantial savings in production time and cost maybe achieved.

In another embodiment, a method of wafer patterning to reduce the sizeof the edge exclusion zone includes steps of:

(a) providing a wafer on which a film has been deposited;

(b) exposing an annular area in an edge exclusion zone of the wafer toradiation having a wavelength suitable for patterning the film in theannular area; and

(c) modulating the radiation while exposing the annular area to form apattern in the film in the annular area.

FIG. 17 illustrates a flow chart of a method of wafer patterning toreduce the edge exclusion zone.

Step 1702 is the entry point of the flow chart 1700.

In step 1704, a wafer is provided on which a film has been deposited.The wafer may be, for example, a silicon wafer used for the productionof integrated circuit die. The film may be, for example, a photo resistfilm used in conjunction with photolithography equipment to maskspecific areas of the wafer during various processing steps of the die.

In step 1706, an annular area in an edge exclusion zone of the wafer isexposed to radiation having a wavelength suitable for removing the filmin the annular area. An example of a radiation having a suitablewavelength is ultraviolet light.

In step 1708, the radiation is modulated while exposing the annular areato form a pattern in the film in the annular area. The modulation maybe, for example, a temporal modulation produced by a strobe or arotating shutter that produces a pattern of radial lines. Alternatively,a spatial modulation may be used to produce a pattern of circlesconcentric with the center of the wafer, and a combination of temporaland spatial modulation may be used to produce a checkerboard pattern.After exposing the annular area in the edge exclusion zone to form thepattern, the pattern is developed according to well known techniques toremove the portion of the film in the edge exclusion zone that is notincluded in the pattern.

Step 1710 is the exit point of the flow chart 1700.

Although the method illustrated by the flowchart description above isdescribed and shown with reference to specific steps performed in aspecific order, these steps may be combined, sub-divided, or reorderedwithout departing from the scope of the claims. Unless specificallyindicated herein, the order and grouping of steps is not a limitation ofother embodiments that may lie within the scope of the claims.

The exemplary embodiments described above assume that a positive resistis used to remove the exposed areas from the edge exclusion zone to formthe pattern. The same method may be used with negative resist, exceptthat the unexposed areas are removed from the edge exclusion zone toform the pattern. Other techniques for exposing resist such as electronbeam or laser based direct write may be used to pattern the edgeexclusion zone to practice other embodiments within the scope of theappended claims.

The specific embodiments and applications thereof do not precludemodifications and variations made thereto by those skilled in the artwithin the scope of the following claims.

1. A method comprising steps of: (a) providing a wafer on which a filmhas been deposited; (b) exposing an annular area in an edge exclusionzone of the wafer to radiation having a wavelength suitable forpatterning the film in the annular area; and (c) modulating theradiation while exposing the annular area to form a pattern in the filmin the annular area.
 2. The method of claim 1 wherein the film is aphoto resist film.
 3. The method of claim 1 wherein step (c) comprisesat least one of spatial modulation and temporal modulation.
 4. Themethod of claim 3 wherein step (c) comprises creating a pattern ofcircles.
 5. The method of claim 3 wherein step (c) comprises creating apattern of lines.
 6. The method of claim 1 wherein step (b) comprisesrotating the wafer.
 7. The method of claim 1 wherein step (b) comprisesrotating a source of the radiation around the wafer.
 8. The method ofclaim 1 wherein step (b) comprises exposing the annular area toultraviolet radiation.
 9. The method of claim 1 further comprising astep of developing the exposed annular area to remove a portion of thefilm that is not included in the pattern.